Closed loop control systems are employed to maintain a process variable of a system under control at a desired set point. For example, the speed of a motor is a process variable that is controlled to maintain the speed at the set point such as described in commonly owned U.S. Pat. No. 6,690,989. During operation, the process variable may differ from the set point due to various causes including environmental disturbances and changes in the physical characteristics of the system under control. The closed loop control system continuously measures the process variable via a feedback signal, and the closed loop control system generates a correction based on the deviation of the process variable of the feedback signal. The correction is intended to force the process variable back to the set point.
While the closed loop control system is effective at preventing the process variable from straying far from the set point, periodic disturbances may be present within the feedback signal that cause problems for the closed loop control system. These periodic disturbances may include actual periodic disturbances of the process variable that are not effectively handled by the closed loop control system and may also include periodic disturbances that appear as deviations from the set point and cause the closed loop control system to generate a correction to counter the deviation. However, the periodic disturbances are a by-product of physical characteristics of the system under control and/or of the sensors used to produce the feedback signal, and these periodic disturbances may or may not be actual deviations of the process variable. Therefore, when the correction is made, the result is that the process variable may not be as desired.